3.7 Mitosis

Question 1

Summarise the stages of the cell cycle in eukaryotic cells

Answer 1

Stage 1:Interphase
● DNA replicates semi-conservatively (S phase)
○ Leading to 2 chromatids (identical copies) joined at a centromere
● Number of organelles & volume of cytoplasm increases, protein synthesis (G1 / G2)

Stage 2:Mitosis
● Nucleus divides
● To produce 2 nuclei with identical copies of DNA produced by parent cell

Stage 3:Cytokinesis
● Cytoplasm and cell membrane (normally) divide
● To form 2 new genetically identical daughter cells

Question 2

Describe the behaviour of chromosomes & role of spindle fibres in mitosis
What happens in stage 1(4) and 2 (2) of mitosis

Answer 2

Stage 1:Prophase
● Chromosomes condense, becoming shorter / thicker so visible
○ Appear as 2 sister chromatids joined by a centromere
● Nuclear envelope breaks down
● Centrioles move to opposite poles forming spindle network
● Spindle fibres start to attach to chromosomes by their centromeres

Stage 2:Metaphase
● Spindle fibres attach to chromosomes by their centromeres
● Chromosomes align along equator

Question 3

Describe the behaviour of chromosomes & role of spindle fibres in mitosis
What happens in stage 3 and 4 of mitosis (6)

Answer 3

Stage 3: Anaphase
● Spindle fibres shorten / contract
● Centromere divides
● Pulling chromatids (from each pair) to opposite poles of cell
Requires ATP

Stage 4:Telophase
● Chromosomes uncoil, becoming longer / thinner
● Nuclear envelopes reform = 2 nuclei
● Spindle fibres / centrioles break down

Question 4

Describe how prok cells replicate (3)

Answer 4

Binary fission:
1. Replication of circular DNA
2. Replication of plasmids
3. Division of cytoplasm to produce 2 daughter cells
○ Single copy of circular DNA
○ Variable number of copies of plasmids

Question 5

Why do some Euk cells not undergo the cell cycle

Answer 5

● Within multicellular organisms, not all cells retain the ability to divide (eg. neurons)
● Only cells that do retain this ability go through a cell cycle
Nuerons do not have centrioles

Question 6

What is the importance of Mitosis in the life of an organism(4)

Answer 6

Parent cell divides to produce 2 genetically identical daughter cells for:
● Growth of multicellular organisms by increasing cell number
● Replacing cells to repair damaged tissues
● Asexual reproduction

Question 7

Describe how viruses replicate (3)

Answer 7

1. Attachment proteins attach to complementary receptors on host cell
2. Inject viral nucleic acid (DNA/RNA) into host cell
3. Infected host cell replicates virus particles:
   a. Nucleic acid replicated
   b. Cell produces viral protein / capsid / enzymes
   c. Virus assembled then released

Question 8

Features of mitosis

Answer 8

one round of divison
Genetically identical cells
Dipoloidcells
Growth and repair-e.g. clonal expansion of B cells